The present invention relates to a system and method for forming a composite fiber-reinforced polymeric structure. More particularly, the present invention relates to an apparatus and method for forming a composite fiber-reinforced polymeric structure having at least two polymeric sheets in opposed spaced relationship such that a cavity is formed therebetween, a fibrous reinforcing layer bonded to the inner surface of at least one of the polymeric sheets, and a polymeric core material introduced under vacuum into the cavity between the polymeric sheets. The polymeric core material exhibits a resinous character in the region of the fibrous reinforcing layer to impregnate the fibrous layer and adhere the fibrous layer to the adjacent polymeric sheet, and exhibits a foamed character defined by a multiplicity of foam cells in the region on the opposite side of the fibrous layer relative to the polymeric sheet. The present invention is particularly advantageous for forming relatively large composite structures, such as structural components for automobiles, trucks, recreational vehicles, and boats.
Composite structures comprising polymeric outer layers and fiber-reinforced foam cores are known in the prior art. For example, U.S. Pat. No. 4,910,067 assigned to the Assignee of the present invention (xe2x80x9cthe ""067 patentxe2x80x9d), discloses a structural composite comprising polymeric outer layers, a layer of fibrous material, and a foam core. It also has been known in the prior art to manufacture this type of composite structure with two polymeric layers, two fibrous layers wherein each fibrous layer is adhesively attached to an inner wall of a respective polymeric layer, and the foam core disposed within the space between the fibrous layers. The polymeric material of the foam core exhibited both a resinous and a foaming character, such that the resinous core material penetrated the fibrous layers, and the foamed core material filled the space between the fibrous layers.
The ""067 patent further discloses a method of manufacturing a structural composite comprising the steps of: forming a polymeric layer into a desired shape; treating the surface of the polymeric layer by etching and oxidation; transferring the polymeric layer to a molding surface of a mold; adhesively attaching a layer of fibrous reinforcement to an opposing molding surface of a mold; mating the molding surfaces within the mold to form a cavity therebetween; injecting a foamable polymer into the cavity; permitting the foam to expand and thereby form a fiber-reinforced polymeric composite structure; and curing the structure in the mold. Alternatively, in order to promote the penetration of the fibrous reinforcement by the foam in a resinous state, the ""067 patent further discloses that the layer of fiber can be treated with a defoaming agent capable of converting the foamable polymer to a liquid.
One drawback associated with these prior art structural composites, and methods of manufacturing such structural composites, is that the relatively viscous core materials cannot rapidly fill the cavity formed between the outer polymeric layers, and moreover, cannot rapidly and fully penetrate or impregnate the fibrous layers. Accordingly, such prior art structural composites have employed only relatively lightweight, unidirectional fibrous layers, that can be more easily penetrated (or xe2x80x9cwetted outxe2x80x9d) by the relatively viscous core materials in comparison to heavier, multi-directional fiber reinforcement layers. As a result, such prior art composite structures tend to be relatively weaker than otherwise desired and cannot be used to form primary structural parts or components. In addition, such prior art composite structures and methods have not proven to be cost effective for manufacturing parts in substantial quantities due to the relatively high cycle times required to allow the foam to expand, fill the core, and penetrate the fibrous layers.
Several other methods are known for manufacturing structural composites in various sizes and volumes for use in a number of technical fields and industries, including the automotive, marine, agricultural and recreational machinery, construction and manufactured housing, and industrial enclosure fields and industries. For example, U.S. Pat. No. 5,588,392 to Bailey shows a resin transfer molding process for manufacturing a fiber-reinforced plastic boat hull; U.S. Pat. No. 5,853,649 to Tisack et al. shows a method for manufacturing an interior automotive foam panel using a radio frequency electric field to promote bonding of the foam to the substrate; and U.S. Pat. No. 5,972,260 to Manni shows a process for vacuum forming polyurethane mixed with a pentane blowing agent to manufacture flat insulating panels.
Each process described above and elsewhere in the prior art is uniquely suited for distinctively different segments of various markets based upon the size of the finished part and the volume of demand for the finished part. Some processes are uniquely suited for producing large parts in low volumes, while other processes are uniquely suited for producing small parts in high volumes. As production volumes increase, the complexity of the machinery involved, and the corresponding pressure applied to that machinery, necessarily increases. Accordingly, when employing these prior art processes, the size of a part that can be formed in relatively high volumes correspondingly decreases because of the processing difficulties associated with molding relatively large parts under relatively higher pressures.
For example, it is known in the prior art to employ a fiberglass xe2x80x9cspray-upxe2x80x9d technology to form large parts having surface areas in the range of about 50-200 square feet. However, this technology has not proven to be economically feasible for producing high volumes of parts, such as in excess of 5,000 parts. Instead, resin transfer molding frequently has been used in the prior art to form relatively smaller parts in relatively higher volumes. For example, resin transfer molding typically has been used to manufacture parts having surface areas in the range of about 5-50 square feet, and in volumes of about 5,000-20,000 parts. Similarly, compression molding has been used in the prior art to form relatively smaller parts in relatively higher volumes. For example, compression molding typically has been used to manufacture parts having surface areas less than about 10 square feet, and in volumes of about 25,000-50,000 parts. To form parts in volumes greater than 50,000, the prior art typically has employed injection molding processes. Such processes, however, are generally limited to producing relatively smaller parts in comparison to the above-described processes.
Accordingly, one drawback associated with these and other prior art processes for manufacturing structural composites is the inability to manufacture relatively large parts, such as parts having surface areas greater than about 25 square feet, in relatively high volumes, in a commercially feasible manner.
Another drawback associated with these and other prior art methods for manufacturing structural composites, particularly fiber-reinforced polymeric composites with foam cores, is the difficulty in forming relatively large, thin-walled products that retain the composite""s strength as well as a high-grade, cosmetic, impact and chemical resistant, weatherable exposed surface.
Accordingly, it is an object of the present invention to overcome one or more of the above described and other drawbacks and disadvantages of the prior art, and to provide a system and method that may be employed to form relatively large composite structures in relatively high volumes while exhibiting reduced cycle times and improved strength.
The present invention is directed to a system and method for making a composite structure in a mold having opposing mold surfaces for receiving the composite structure therebetween. The composite structure of the invention includes at least two outer polymeric layers spaced apart from each other and defining a cavity therebetween, a foam core located between the two polymeric layers and made of a core material capable of exhibiting a foamed character and a resinous character, at least one fibrous layer located between a respective one of the polymeric layers and the foam core, and at least one resinous layer made of the core material and located between an outer polymeric layer and respective fibrous layer. A cavity is formed between the outer polymeric layers, and is defined by at least one first region extending between each fibrous layer and a respective outer polymeric layer, and at least one second region located between the fibrous layers.
The two outer polymeric layers are disposed between the opposing mold surfaces and are spaced apart from each other to define the cavity therebetween. At least one fibrous layer, and preferably two, is provided between the two outer polymeric layers to create the first and second regions of the cavity. Preferably, the fibrous layers are adhesively attached to the respective polymeric layers by, for example, a radiation-activated adhesive, prior to placing the polymeric layers and fibrous layers in the opposing surfaces of the mold. The cavity formed between the polymeric layers is evacuated to create a predetermined vacuum therein, and the core material is introduced in a resinous character into the second region of the evacuated cavity. A blowing agent of the core material is then activated by subjecting the core material to the vacuum within the cavity, and the core material in the second region of the cavity is, in turn, converted from a resinous character to a foamed character. Upon substantially filling the second region of the evacuated cavity with the foamed core material, the foamed core material that contacts the fibrous layers is then converted from a foamed character to a substantially resinous character to create a relatively dense, resinous interface between each fibrous layer and the foamed core. A catalytic reaction is initiated within the foamed core to cure the foamed core, and negative pressure gradients are then created in the direction from the foamed core toward the fibrous layers. Preferably, the negative pressure gradients are created by maintaining the vacuum in the first regions of the cavity between the fibrous layers and the outer polymeric layers, and increasing the pressure in the foamed core through the catalytic reaction of the core material. The negative pressure gradients are used to cause the resinous core material at the interface of each fibrous layer and the foamed core to penetrate the fibrous layers and, in turn, substantially fill the first regions of the cavity with the resinous core material. The resinous core material is then cured in the first regions of the cavity to fixedly attach the resinous core material and fibrous layers to the outer polymeric layers.
One advantage of the present invention is that the vacuum and relatively low-viscosity of the polymeric core material allows the material to rapidly fill the cavity. Then, the vacuum in combination with the negative pressure gradients created by the catalytic reaction in the foamed core, cause the resinous core material to rapidly impregnate the fibrous material, fill the first regions of the cavity, and bond the fibrous material to the polymeric sheet(s). As a result, the cycle times required to manufacture relatively large composite structures are significantly reduced in comparison to that of the above-described prior art processes and structures. Yet another advantage of the present invention is that the vacuum in combination with the preferred, relatively low viscosity core materials, allow the resinous core to rapidly and fully impregnate (or xe2x80x9cwet outxe2x80x9d) the fiber reinforcement layers, and thereby create significantly stronger structures, having significantly improved strength-to-weight ratios in comparison to the above-described prior art structures. The apparatus and method of the present invention are therefore particularly advantageous for forming relatively large, thin-walled composite structures, in high volumes and in a commercially feasible manner, that exhibit improved strength in comparison to the above-mentioned prior art composite structures, as well as high-grade, cosmetically-appealing, impact and chemical resistant, and/or weatherable exposed surfaces. Accordingly, the apparatus and method of the present invention are particularly useful for manufacturing components for automobiles and trucks, including, for example, tonneaus for pick-up trucks, hard tops for automobiles and sports utility vehicles (xe2x80x9cSUVsxe2x80x9d), and other relatively large parts for trucks, vans and recreational vehicles.
Other advantages of the present invention will become apparent in view of the following detailed description and accompanying drawings.